LibGUI: Expose a function to clear GTextEditor selection

Author: https://github.com/gootik
Commit: 5c2ef01f7b
Pull-request: https://github.com/SerenityOS/serenity/pull/835

Libraries/LibGUI/GTextEditor.cpp

@@ -1223,6 +1223,14 @@ void GTextEditor::set_selection(const GTextRange& selection)
     update();
 }
 
+void GTextEditor::clear_selection()
+{
+    if (!has_selection())
+        return;
+    m_selection.clear();
+    update();
+}
+
 void GTextEditor::recompute_all_visual_lines()
 {
     int y_offset = 0;